In the presence of 1-methoxy PMS, WST-8 can be reduced to highly water-soluble orange-yellow formazan by intracellular dehydrogenase. The amount of the formazan generated is proportional to the number of living cells. Therefore, the number of living cells can be measured by measuring the absorbance of the formazan at 450 nm. CCK-8 Reagent of this kit is a ready to use reagent, which can be directly added to the cell samples, and can be detected after incubation for a certain period of time without pre-preparing various components.